1. Field of the Invention
The invention relates to a short-arc, ultra-high pressure discharge lamp in which the mercury vapor pressure during operation is at least 150 atm. The invention relates especially to a short-arc, ultra-high pressure discharge lamp which is used as the light source of a liquid crystal display device and a projector device using a DMD (digital mirror device), like a DLP (digital light processor) or the like.
2. Description of Related Art
In a projector device of the projection type noted above, there is a demand for uniform illumination of images onto a rectangular screen with sufficient color reproduction. Thus, the light source is a metal halide lamp which is filled with mercury and a metal halide. Furthermore, recently, smaller and smaller metal halide lamps and more and more often spot light sources have been produced and lamps with extremely small distances between the electrodes have been used in practice.
Against this background, recently, instead of metal halide lamps, lamps with an extremely high mercury vapor pressure, for example, 150 atm, have been proposed. Here, the increased mercury vapor pressure suppresses broadening of the arc (the arc is compressed) and a major increase of the light intensity is desired. One such ultra-high pressure discharge lamp is disclosed in JP-OS HEI 2-148561 (corresponds to U.S. Pat. No. 5,109,181) and JP-OS HEI 6-52830 (corresponds to U.S. Pat. No. 5,497,049).
In such an ultra-high pressure discharge lamp, the pressure within the arc tube during operation is extremely high. Therefore, in the side tube parts which extend from opposite sides of the arc part, it is necessary to arrange the silica glass comprising these side tube parts, the electrodes and the metal foils for power supply sufficiently tightly and directly adjoining one another. When they do not adjoin one another tightly enough, the added gas leaks or cracks form. Therefore, in the process of hermetic sealing of the side tube parts, the silica glass is heated, for example, at a high temperature of 2000° C., and in this state, the silica glass with a great thickness is gradually subjected to shrinking or a pinch seal. In this way, the adhesive property of the side tube parts is increased.
However, if the silica glass is heated up to an excessively high temperature, the disadvantage occurs that, after completion of the discharge lamp, the side tube parts are damaged, even if the adhesion of the silica glass to the electrodes or metal foils is increased.
It can be imagined that the cause of this disadvantage is the following:
After heat treatment, in the stage in which the temperature of the side tube parts is gradually reduced, as a result of differences between the coefficient of expansion of the material (tungsten) comprising the electrodes, and the coefficient of expansion of the material (silica glass) comprising the side tube parts, there is a relative difference in the amount of expansion. This causes the formation of cracks in an area in which the two come into contact with one another.
In order to eliminate this disadvantage, the arrangement shown in FIG. 8 was proposed. Here, the arrangement of the discharge lamp is shown schematically. The light emitting part 2 adjoins the side tube parts 3 in which an electrode 6 (the upholding part 6a of the electrode) or an electrode 7 (the upholding part 7a of the electrode) are each connected to the metal foil 8. A coil component 10 is wound around the upholding parts 6a, 7a of the electrodes which have been installed in the side tube parts 3. This arrangement reduces the stress which is exerted on the silica glass as a result of the thermal expansion of the upholding parts 6a, 7a of the electrode by the coil component 10 which has been wound around the upholding parts 6a, 7a of the electrode. This arrangement is described, for example, in Japanese patent disclosure document HEI 11-176385.
But in reality, there was the disadvantage that cracks remain in the vicinity of the upholding parts 6a, 7a of the electrode and the coil component 10 even if the thermal expansion of the upholding parts 6a, 7a of the electrode is relieved by this arrangement. These cracks are initially extremely small, but there are often cases in which they lead to damage of the side tube parts 3 when the mercury vapor pressure of the light emitting part 10 is roughly 150 atm. Furthermore, in recent years, there has been a demand for a higher mercury vapor pressure of 200 atm and beyond to 300 atm. At such a high mercury vapor pressure, the growth of cracks is accelerated during lamp operation. As a result, there is the disadvantage that damage of the side tube parts 3 clearly occurs. This means than the cracks gradually become larger during operation of a lamp with a high mercury vapor pressure, even if the cracks were extremely small at the start. It can be stated that preventing this problem is a new technical task since the problem never occurred with a conventional mercury lamp with a vapor pressure during operation of at most roughly 50 atm.